A ferrite bead or ferrite choke is a passive electric component that suppresses high frequency noise in electronic circuits. It is a specific type of electronic choke. Ferrite beads employ high frequency current dissipation in a ferrite ceramic to build high frequency noise suppression devices. Ferrite beads may also be called blocks, cores, rings, EMI filters, or chokes.
Ferrite beads prevent interference in two directions: from a device or to a device. A conductive cable acts as an antenna – if the device produces radio frequency energy, this can be transmitted through the cable, which acts as an unintentional radiator. In this case the bead is required for regulatory compliance, to reduce EMI. Conversely, if there are other sources of EMI, such as household appliances, the bead prevents the cable from acting as an antenna and receiving interference from these other devices. This is particularly common on data cables and on medical equipment.
Large ferrite beads are commonly seen on external cabling. Various smaller ferrite beads are used internally in circuits—on conductors or around the pins of small circuit-board components, such as transistors, connectors and integrated circuits.
Ferrite beads are used as inductors to form a passive low-pass filter. The geometry and electromagnetic properties of coiled wire over the ferrite bead result in an impedance for high-frequency signals, attenuating high frequency EMI/RFI electronic noise. The energy is either reflected back up the cable, or dissipated as low level heat. Only in extreme cases is the heat noticeable.
A pure inductor does not dissipate energy but it produces reactance that impedes the flow of higher frequency signals. This reactance is commonly referred to simply as impedance, although impedance can be any combination of resistance and reactance.
A ferrite core or bead can be added to an inductor to improve, in two ways, its ability to block unwanted high frequency noise. First, the ferrite concentrates the magnetic field, increases inductance and therefore reactance, which impedes or ‘filters out’ the noise. Second, if the ferrite is so designed, it can produce an additional loss in the form of resistance in the ferrite itself. The ferrite creates an inductor with a very low Q factor. This loss heats the ferrite, but normally it is a negligible amount of heat. While the signal level is large enough to cause interference, or undesirable effects in sensitive circuits, the energy blocked is typically quite small. Depending on the application, the resistive loss characteristic of the ferrite may or may not be desired.
A design that uses a ferrite bead to improve noise filtering must take into account specific circuit characteristics and the frequency range to block. Different ferrite materials have different properties with respect to frequency, and manufacturer's literature helps select the most effective material for the frequency range.
Ferrite beads are one of the simplest and least expensive types of interference filters to install on preexisting electronic cabling. For a simple ferrite ring, the wire is simply wrapped around the core through the center, typically five or seven times. Clamp-on cores are also available, which attach without wrapping the wire: this type of ferrite core is usually designed so that the wire passes only once through it. If the fit is not snug enough, the core can be secured with cable ties or, if the center is large enough, the cabling can loop through one or more times. Small ferrite beads can be slipped over component leads to suppress parasitic oscillation.
Surface-mount ferrite beads are available. These are soldered into a gap in the printed circuit board trace, just like any other surface mount inductor. Inside the bead component, a coil of wire runs between layers of ferrite to form a multi-turn inductor around the high permeability core.